An infrared gas detector such as the one disclosed in JP-A-2001-228086 and as shown in FIG. 3 is known. Such a gas detector is equipped with an incandescent electric bulb 1 as an infrared light source, infrared sensing elements 2a and 2b that form an infrared sensor, and a gas cell 4 to which is introduced a target gas to be measured. The target gas is introduced inside the gas cell 4 through vent holes 5 provided in the gas cell 4. The incandescent electric bulb 1 radiates infrared light of a broad wavelength. The infrared light propagates through the target gas introduced into the gas cell 4 and is incident on the infrared sensing elements 2a and 2b after passing through a multi-wavelength selection filter and a band pass filter 3. The infrared sensing elements 2a and 2b sense the infrared light at a desired wavelength, which is adjusted by the multi-wavelength selection filter and the band pass filter 3, and output detection signals corresponding to the intensity of the received infrared light.
As the infrared light propagates through the target gas, infrared light at a certain wavelength is absorbed by the target gas. The intensity of the infrared light that is absorbed by the target gas varies in accordance with the concentration of the target gas, and the wavelength at which the infrared light is absorbed is dependent on the class of the target gas. Therefore, by adjusting the multi-wavelength selection filter and the band pass filter 3 based on the particular target gas, the output signals of the infrared sensing elements 2a and 2b may correspond to the absorption characteristics of the infrared light, thereby enabling the concentrations of the target gases to be detected.
As shown in FIG. 3, the gas cell 4 is provided with a concave paraboloidal reflector 6. The concave paraboloidal reflector 6 reflects infrared light scattered in the direction opposite the sensing elements 2a and 2b, and thereby directs the infrared light in the direction of the sensing elements 2a and 2b. 
However, in the case of the above infrared gas detector, the rays of the infrared light emitted from the incandescent electric bulb 1 become parallel to the side wall of the cylindrical gas cell 4 as a result of being reflected by the concave paraboloidal reflector 6, and irradiate the infrared sensing elements 2a and 2b. When the rays are parallel to the side wall of the gas cell 4, some of rays irradiate regions of the detector other than where the infrared sensing elements 2a and 2b are disposed, and therefore efficient radiation of infrared light from the incandescent bulb 1 cannot be obtained.